Preheat oven to 425 degrees F. Line a 12-cup muffin tin with muffin papers and set aside.
In a large bowl, combine flour, baking powder, sugar and salt.
In another large bowl, whisk together the eggs, butter, oil, orange zest, orange juice, buttermilk and vanilla. Add dry ingredients to the wet ingredients and stir until just combined. Fold in the chocolate chunks. Divide evenly among the muffin cups (I like to use an ice cream scoop to make sure they are even).
Bake for 13-15 minutes, until a toothpick comes out clean.